Speed sensors, or detectors of various types are well known. In recent years the application of speed detectors to automotive display and automotive control functions has stimulated increased demands on and sophistication, of those sensors. In one common configuration, the speed sensor is placed in non-contacting relation with a member having reference points or teeth, the relative motion therebetween is detected by the sensor and associated circuitry provides at least timing reference signals and may ultimately provide an indication of the relevant speed. Usually, the sensor is fixedly positioned and the moving member is a toothed wheel, or so called "tone wheel", which rotates at a speed which is a known function of the speed to be measured.
One often used class of speed sensors employs one or more sense coils positioned in proximity with the rotating teeth of a tone wheel for exploiting either magnetic or eddy current effects depending on whether or not magnetic materials are present in the teeth.
One sensor of the aforementioned general type is disclosed in U.S. Pat. No. 3,716,787 by Peter W. Hammond. In that reference, there is disclosed a speed monitoring device having a phase measurement circuit which is utilized to measure the changes in the inductance of a sensor coil as affected by the proximity of magnetically distinct pins on a tone wheel. The relative increase and decrease in spacing between tone wheel pins, or teeth, and the spaces between such teeth with respect to the sensor coil serves to vary the impedance or inductance of that coil in a tuned circuit in which it is included. Such variation in the impedance effects a circuit phase-shift with respect to a predetermined reference phase of voltage and current in the circuit to indicate movement of the wheel and a measurement of the speed of movement in a measured time interval.
Another U.S. Pat. No. 3,750,128 to Said Sapir discloses a pulse generator which produces output pulses at a pulse repetition frequency directly proportional to the wheel velocity. The output pulses may be used in a conventional anti-skid braking system. The pulse generator is operative even at low velocities since it employs an oscillator-energized variable reluctance transformer.
Yet another device for sensing speed is disclosed in U.S. Pat. No. 3,728,565 to Gerald O'Callaghan. That reference discloses first and second spaced apart stator windings positioned near a rotating tone wheel. Respective sinusoidal voltages are induced in each of the stator windings and the magnitude of those voltages is indicative of the speed.
With respect to the U.S. Pat. No. 3,728,565 reference, it will be appreciated that the system is dependent upon the speed of rotation of the tone wheel for an amplitude signal to provide a corresponding speed signal. Such systems are inherently limited, particularly with respect to low speeds of operation and/or variations in the signal magnitude occasioned by other than speed alone. It is also desirable that the sensor operate over a relatively wide gap between it and the tone wheel, however, amplitude-dependent systems exhibit characteristic weaknesses in that regard. Though the U.S. Pat. Nos. 3,716,787 and 3,750,128 references are not dependent upon some minimum speed of the tone wheel for operability since they rely upon phase-shift techniques, they do possess other limitations. For instance, with respect to U.S. Pat. No. 3,716,787, the phase shift in the circuit containing the sensing coil is determined relative to a fixed reference signal from the oscillator which drives the sensing coil circuit. Since the reference signal has a fixed frequency, the circuit containing the sensing coil is capable of only a limited relative phase shift. Such limitation generally requires a relatively strong interaction between the tone wheel and the coil and circuitry to provide a desired response. In U.S. Pat. No. 3,750,128 a transformer interacts with the tone wheel and with a rectangular wave generator to create the requisite phase shifts.
One recent inductive speed sensor which has been developed to overcome some of the short-comings of the aforementioned prior art is disclosed and claimed in the aforementioned companion application Ser. No. 07/240,782 (HCI-386) of Welcome and Sparks filed on even date herewith and assigned to the same assignee as the present application. The disclosure of that application is incorporated herein by reference. That inductive speed sensor employs two coils, typically each associated with respective iron cores, for magnetically interacting with the teeth and slots of a tone wheel for speed detection. In the event the teeth are of a nonferromagnetic material, the interaction is based on eddy current principles.
The circuitry associated with those two coils comprises a phase sense oscillator incorporating one of the coils and a tuned sensor network incorporating the other coil. The frequency of the phase sense oscillator is varied as a function of the instantaneous inductance of its respective coil, which is in turn determined by its present positioning relative to the teeth and slots of the tone wheel. Although the phase sense oscillator drives the tuned sensor network bearing the sensor coil, that second coil is displaced from the first such that the inductive changes to it as a result of the passing teeth and slots are normally out-of-phase with those affecting the first coil. The resulting signals from the phase sense oscillator and the tuned sensor network are typically phase shifted by varying amounts, however, that phase will reverse itself twice during rotation of the tone wheel through an angle .theta. commensurate with the pitch between successive teeth on the tone wheel. Further, phase condition detection circuitry associated with the inductive speed detector monitors the signals provided by the phase sense oscillator and the tuned sensor network to detect the instant at which the relative phases of the two signals reverse relative to one another. That event is indicative of some fixed position on the tone wheel and accordingly, is repeatable as the tone wheel rotates for generating timing reference pulses.
A potential problem associated with various inductive speed detectors, and particularly those relying on phase shift principles, resides in the possible generation of unwanted output timing reference signals as a result of a small amount of reverse rotation, or backlash, of the tone wheel which may be generated by mechanical vibration, either when the wheel is in a nonrotating state or is rotating at an extremely slow speed. Although that reverse rotation may be of relatively small amplitude, it may nonetheless be sufficient to be detected as a reversal of the phase sequence between the signals and thus be incorrectly interpreted as a valid timing reference pulse.
Although the aforementioned U.S. Pat. No. 3,716,787 does disclose the provision of a hysteresis effect in its particular circuit to overcome some of the foregoing limitations, it does so by use of a feedback arrangement which relies upon scaling resistors to establish a threshold level against which the oscillating signal derived from the signal sensor is compared in the generation of a digital signal level for controlling the output flip-flop device. While the provision of hysteresis in that manner for that particular configuration of sensing circuit may be acceptable, it is preferable to minimize the number of components in the hysteresis circuit which may experience variances in their tolerances. Moreover, it is desirable to provide an effective and optimal hysteresis control for a speed detector of the type employing two inductive sensing coils and respective associated circuitry.
Accordingly, it is a principal object of the invention to provide an improved speed detector apparatus which is operative at all speeds, yet is relatively insensitive to some vibration in the tone wheel at low speed conditions. Included within this object is the provision of a hysteresis effect in a manner which is relatively economical and reliable, particularly in a circuit employing two sensor coils.